1. Field of the Invention
This invention relates generally to devices for detecting blank or unrecorded portions of a record medium. This invention is more particularly directed to a device for detecting, on a record medium such as a phonograph disc, the positions of unrecorded interband regions, such as interband gaps or rings, which are disposed between bands of recorded information on the record medium. This invention is more specifically directed to such a device which detects differences in the reflectivity of the interband regions and the bands of recorded information by emitting light or other electromagnetic radiation to be incident on the recorded surface of the medium, and then detecting the level of the light that is reflected thereby.
2. Description of the Prior Art
As is commonly known, phonograph discs are formed of a disc substrate having substantially concentric spiral grooves thereon containing audio information, such as music or speech. Typically, the audio information is represented by undulations and variations in the size and depth of the grooves, and the recorded music or speech is picked up by a phonograph stylus guided in the spiral grooves. In a so-called long-playing (LP) disc or "album", several selections of music or other audio sounds are recorded on each side. In such case, each selection is recorded in a separate annular section formed of a plurality of such substantially concentric spiral grooves arranged together in a so-called band. The bands containing the music or other audio information are separated from one another by blank regions, or interband rings, in which no audio information is recorded. Normally, only a shallow guide groove is provided in each such interband ring for guiding the phonograph stylus from the end of one band to the beginning of the next.
Whe such an LP disc is played, the listener often desires to hear only a certain one of the several selections on one of the album sides. To do this, the position of the interband ring immediately preceding the desired band is determined, and the phonograph tone arm is moved to a position directly thereover. Then the tone arm is lowered so that the stylus engages the groove and led into the desired band, and the desired selection is played.
The order of the selections on each album side is virtually always printed on a record label affixed to the center of that album side. Consequently, the listener can easily determine the particular band to be played in order to hear the desired selection. For example, if the listener desires to hear Chopin's minor piano sonata from a particular LP disc, he or she can determine from the label of such disc that the desired selection is, for example, recorded on the third band. Therefore, in order to play this selection, the listener need only move the tone arm to the interband ring between the second and third bands, and lower the tone arm to engage the guide groove therein.
In other recorded media, such as optical discs, the information can be recorded as a series of pits or lands arranged in concentric spiral grooves. In such discs, several selections can be recorded in respective bands with silent or unrecorded rings or gaps disposed therebetween.
The light-reflecting properties of the above LP discs or other discs are quite different at the bands from the corresponding properties at the rings between bands. In the bands, the grooves are packed closely together and, oweing to the information contained therein, are irregular in shape. Consequently, light incident on the bands tends to be scattered or dispersed. In contrast to this, the interband rings are relatively flat, with only a shallow guide groove therein, and incident light tends to be reflected more or less uniformly.
In view of the above characteristics of LP albums and other recorded media, it has been previously proposed to construct a phonograph record player in which the position of the interband ring, leading into a particular band containing a desired musical selection, can be automatically detected. To provide such a phonograph record player it has been proposed to incorporate an interband region detecting device with which a beam of light is emitted to be incident on the recorded surface of the disc, and with which the intensity of the light reflected thereby it detected. Blank portions, that is, the interband rings, can be readily discriminated on the basis of their greater reflectivity. In other words, the interband region detecting device can, for example, provide output pulses whenever greater amounts of reflected light are received, thus identifying the presence of respective interband rings.
The tone arm, or other movable arm of the phonograph, can be adapted to carry light emitting and detecting elements for the interband region detecting device. In such case the tone arm can be swept at a moderate speed over the surface of an LP disc on the phonograph. The detecting device will emit a pulse each time an interband ring is reached. When the tone arm is swept toward a desired band, these pulses are produced and can be counted, for example, in a digital counter. When a preset count is reached, i.e., when a pulse is produced indicating that the tone arm has swept to the position of the interband ring immediately preceding the desired band, the tone arm is automatically stopped and lowered. Then the desired selection is played.
While such previously proposed interband region detecting device is generally sufficient for use with any one particular disc, there are problems that arise from its use if record discs are interchanged on the record player.
Generally, each record disc has a particular light reflectivity which is determined, for example, by the type and quality of the material, the color and smoothness of the surface, and the conditions existing when the particular disc was pressed. Thus, if light of a given intensity is incident on each of a plurality of various record discs, the light reflected from each disc will have a different respective intensity. Accordingly, in the previously proposed device, means are generally incorporated for manually controlling the sensitivity thereof in accordance with the reflectivity of the particular record disc on the phonograph.
Normally, the previously proposed device includes a photodetector, such as a photo diode or photo transistor, for converting the intensity of the reflected light beam into an output current or voltage level. Because of disc-to-disc variations in reflectivity, as mentioned above, the output level of the photodetector could be either too high or too low to provide an accurate indication of the presence of the interband rings.
This defect, rendering it impossible to accurately detect the interband rings for a variety of record discs, is inherent in the previously proposed device. Thus, to avoid this defect, it has been further proposed to include an output controller in association with the photodetector. Such output controller generally takes the form of a manually adjustable variable resistance, whose setting is to be changed in accordance with the reflective qualities of the various record discs to be played.
Unfortunately, the setting of such output controller is quite troublesome and is not entirely reliable. For example, the reflectivity of any particular disc is generally unknown, and the required manual setting must be established by trial and error. Also, because the setting is manual and not automatic, a listener may easily forget to change the setting when changing the record discs, and the device will misoperate accordingly.
For the above reasons, an improved detecting device has been sought which will automatically take into account disc-to-disc variations in surface reflectivity.